This application relates to a control for a variable speed compressor motor, wherein the compressor may operate at extremely low average speeds for extended periods of time while still maintaining adequate lubrication.
Refrigerant systems are utilized in many applications to condition an environment. In particular, air conditioners and heat pumps are employed to cool and/or heat air entering an environment. The cooling or heating load of the environment may vary with ambient conditions, occupancy level, other changes in sensible and latent load demands, and as the temperature and/or humidity set points are adjusted by an occupant of the environment.
A feature that has been gaining popularity in improving the efficiency of refrigerant systems is the use of a variable speed drive for the compressor motor. Often, the compressor need not be operated at full speed, such as when the cooling load on the refrigerant system is relatively low. Under such circumstances, it might be desirable to reduce the compressor speed, and thus reduce the overall energy consumption of the refrigerant system. Implementation of a variable speed drive is one of the most efficient techniques to enhance system performance and reduce life-cycle cost of the equipment over a wide spectrum of operating environments and potential applications, especially at part-load conditions.
However, compelling reliability concerns set a lower limit to the desirable compressor speed reduction. In particular, inadequate lubrication of the compressor elements may present a problem at low operating speeds. This often occurs as the compressor oil delivery relies on an operation of a pump installed within the compressor, where the oil pump delivery head is strongly affected by the operating speed. If the compressor operating speed is reduced below a certain level, the oil pump cannot generate required pressure head to deliver the oil to the components that need to be lubricated within the compressor. This leads to the inadequate lubrication of those components and subsequent compressor damage. The internal components that are affected the most are the ones located farther away from the pump inlet. Thus, compressors are often rated with a minimum speed (typically 45 Hz) required to ensure adequate compressor lubrication. Even compressors that are specifically designed for variable speed operation, and incorporate special futures (such as a special oil pump) to promote lubrication, often cannot operate below 30 Hz. However, the minimum speed limit established by proper lubrication requirements may still need to be reduced well below 30 Hz to achieve efficient operation at part-load. That is, given the minimum speed limitations, much of the energy efficiency that could be potentially provided by the variable speed drive is essentially eliminated. Thus, there is a need to provide a compressor operating at a lower average speed than what can be currently achievable with current designs.